Solar Facts

The Sun is a low mass star on the outer reaches of the Milky Way galaxy. The Sun is
some 30,000 light years from the center of the Milky Way and lies on one of the spiral
arms. At this distance, ittakes about 250 million years to complete an orbit around
the galaxy. One light year is the distance travelled by light in one year. One light
year is about 10,000 billion kilometers.

Image of the Milky Way galaxy taken from the Earth.

The Andromeda Galaxy is much like our own Milky Way and can be used to illustrate
the approximate location of the Sun in our own galaxy, marked by the circle and arrow.

Temperatures are in degrees
Densities are in milligrams per cubic centimeter

Activity

The Sun is a very active star. No matter when or where you look, the Sun is always
doing something interesting. One of the most dramatic properties of the Sun's activity
is the existence of a solar cycle, which is best seen
in the pattern of sunspots. The graph to the right shows
the behavior of the average number of sunspots over time for the last 300 years. We
see from that this figure that the number of sunspots on the Sun is not constant but
varies significantly with a peak number showing up, approximately every 11 years.
The Sun, thus, has a cycle of behavior which repeats every 11 years.

It is not known exactly what causes this repeatable behavior but processes in the
deep interior of the Sun, which generate magnetic
field, are thought to be responsible. Over the last 30 years scientists have developed
theories to exlpain this behavior and improved observations from telescopes in space
are providing much needed information to help track down the answer to this particular
solar conundrum.

An interesting pattern develops when we ask where on the Sun do the sunspots appear.
The figure on the left is known as a butterfly diagram,
because the pattern looks like the wings of a butterfly. The figure shows the butterfly
pattern for the last 11 solar cycles (each taking 11 years).

The vertical axis shows the latitude on the Sun which we measure just like on the
Earth (equator is at 0, north pole is at 90N, south pole at 90S). This pattern tells
us that at the beginning of a particular solar cycle, sunspots appear mostly at high
latitudes (30N or 30S). As time goes on the sunspots appear nearer and nearer to the
equator, until a new cycle begins and the process starts all over again.

A more dramatic form of solar activity can be found in solar flares and coronal mass
ejections. These are short-lived disruptions of the solar atmosphere which create
fantastic displays the images of the Sun. Coronal mass ejections and flares are the
solar causes of geomagnetic stroms at the Earth which disrupt telecommunications,
satellites and power grids.

Rotation

One of the most obvious signs that the Sun is not just sitting there is its
rotation. The Sun rotates on its axis about once every 26 days. Because the Sun is
made of gas the rotation is quite different from that of, say, a basketball spinning
on your finger. Different parts of the Sun rotate at different speeds with the fastest
rotation rate occurring at the Sun's equator. The higher the latitude (north or south)
the slower rotation. The Sun's polar regions have a rotation period greater than 30
days. This is known as differential rotation and is similar
to the phenomenon which produces the distinct bands on the planet Jupiter.

Evolution

It has often been said that the Sun is a fairly mediocre star in a fairly boring neighborhood
of the Galaxy. This is true if you ignore the fact that the Sun is the only star we
know of which supports life. However, it is still illustrative to compare the Sun
with other stars in order to understand how stars form, evolve and eventually die.
The Sun is about half way through its prime and will not change significantly as a
star for another 5 billion years.

The basic stages in the Sun's life can be summarized as follows:

Gas cloud

Proto-star

Main sequence

Red giant

Planetary nebula

Remnant

1. Gas cloud: Initially the gas and dust which go to
make up stars is in the form of a large dense cloud that is relatively cold (temperatures
around 10 degrees K, or, -440 degrees F). Parts of this gloud start to fragment and
collapse under their own gravity to form blobs called proto-stars.

2. Proto-star: As the blob collapses it starts to heat
up in its center (gravitational energy is being converted into heat). The proto-star
gets hot enough to glow red (temperatures are around 2000-3000 K). The proto-star
continues to condense until nuclear reactions in its core halt the gravitational collapse.

3. Main sequence: The star is stable, burning Hydrogen
gas to Helium in its core through nuclear fusion. The energy released causes the star
to shine. Stars spend about 90% of their active lifetime as main sequence stars.

4. Red giant: As the star runs out of core fuel to "burn"
the stability of the star is disrupted and the core begins to shrink. The star then
starts to turn Helium into Carbon, allowing it to "shine" for a while longer. The
rapid burning of helium causes the outer layers of the star to puff out, cooling the
star and causing it to glow red. It is now a red giant.
Red giants can eject a lot of mass through ``winds''. While a red giant may be much
larger than the main sequence star from which it came it has less mass. The Sun will
spend approximately 250 million years as a red giant.

5. Planetary nebula: The outer layers of the star are
ejected as core continues to shrink. For stars like the Sun this process produces
what is known as a planetary nebula (Strangely, these
nebulae have nothing whatsoever to do with planets). A planetary nebula is defined
as a shell of hot gas that has been expelled from a star going through its late stages
of evolution. The material in the nebula comes from the outer layers of the star,
leaving its core exposed.

6. Remnant: The low mass core continues shrinking to
form a star known as a white dwarf surrounded by the
planetary nebula.

The Sun is currently in its Main Sequence stage, where
its spends most of its active life. About 90% of all stars are in this stage of their
evolution.